1. Field of the Invention
The invention relates in general to a material used in a semiconductor fabrication process, and more particularly, to a photoresist with an adjustable polarized light response and a photolithography process using the photoresist.
2. Description of the Related Art
As the integration of semiconductor devices increases, the resolution of photolithography process becomes increasingly demanding. The analyzable minimum dimension (R) is defined as: R=k1λ/NA (λ is the wavelength, and NA is the numerical aperture of the optical system). From the above equation, it is known that the larger the numerical aperture is, the higher the resolution is. The numerical aperture of the exposure optical system used in the current photolithography process is thus gradually increased.
When the numerical aperture exceeds 0.7, pattern deformation is caused by the following reasons. First of all, the exposure light adopted for the exposure process is the polarized light. The polarized light includes the P-polarized and S-polarized lights perpendicular to each other in electromagnetic polarization direction. For a pattern with a certain orientation, P- and S-polarized lights cause different intensity profiles in a photoresist, and the total intensity that determines the photoresist pattern is the sum of the intensity profiles for both the P- and S-polarized lights.
When the numerical aperture is smaller than 0.7, the transmission coefficients for the P-polarized light and the S-polarized light are the same. Whatever the orientation of the pattern is, the total intensity profile and the photoresist pattern are not varied. However, when the numerical aperture is larger than 0.7, the transmission coefficient of the P-polarized light is larger than that of the S-polarized light, and the difference of transmission coefficient increases as the numerical aperture increases. Consequently, as the pattern orientation changes, the total intensity profile and the pattern profile are not consistent. An example of this phenomenon is given as follows.
FIGS. 1, 2A and 2B show the intensity profile and total intensity profile of a photoresist for a P-/S-polarized light traveling through a X-/Y-directional pattern and the pitch of a corresponding photoresist pattern (a positive photoresist is adopted). As shown in FIG. 1, the P-polarized light and the S-polarized light are polarized in the X- and Y-directions, respectively. The photomask 100 has a Y-directional pattern 102 and an X-direction pattern 104 with the same pitch (a).
As shown in FIG. 2A, the Y-directional pattern 102 is in the same direction as the polarization direction of the S-polarized light, so that the distribution of the intensity profile 202s of the S-polarized light 202s is narrower than distribution of the intensity profile 202p of the P-polarized light. On the other hand, as the transmission coefficient of the P-polarized light is larger than that of the S-polarized light, the integration of the intensity profile 202p is larger than that of the intensity profile 202s. That is, the total intensity profile 212 of the Y-directional pattern 102 is determined by the wider intensity profile 202p. 
As shown in FIG. 2B, since the X-directional pattern is in the polarization direction of the P-polarized light, the distribution of intensity profile 204p of the P-polarized light is narrower than the distribution of the intensity profile 204s of the S-polarized light. In other words, since the transmission coefficient of the P-polarized light is larger than that of the S-polarized light, the integration of the intensity profile 204p is thus larger than that of the intensity profile 204s. Simply speaking, the total intensity profile 214 is determined by the intensity profile 204p with a narrower distribution.
Referring to FIGS. 2A and 2B, the total intensity profile 212 of the Y-directional pattern 102 is determined by the wider intensity profile 202p, and the total intensity profile 214 of the X-directional pattern 104 is determined by the narrower intensity profile 204p. Therefore, the total intensity profile 212 is larger than the total intensity profile 214. As a result, when a positive photoresist is used, under a certain threshold exposure intensity Eth, the photoresist pattern pitch bX of the X-directional pattern 104 is smaller than the photoresist pattern pitch bY of the Y-directional pattern 102.
To resolve the above deviation, an optical system with a high numerical aperture is used to correct before performing the photolithography process. However, the current optical proximity correction model is designed to calculate the scalar of the incident only. The vector of the incident light (P/S polarized light) is not considered. Therefore, the difference in intensity profile caused by difference of transmission coefficient for P-/S-polarized light and pattern orientation cannot be compensated. The pitch and size of the resultant pattern is varied by the orientation change, so that deviation of different ratios occurs.